In general, an outboard engine system includes an engine room in which an engine is accommodated, and a case member extending downwards from the engine room to accommodate a drive shaft driven by the engine, so that an exhaust gas discharged from the engine is guided downwards within the case member and discharged into water in order to enhance the silencing effect. During idling operation of the engine, a portion of the exhaust gas is diverted and discharged into the air, thereby providing a reduction in back pressure.
There is such a conventionally known outboard engine system described in Japanese Patent Application Laid-open No.8-100625, in which an exhaust passage having a silencing effect is formed by an exhaust gas expansion chamber having an outlet and an inlet.
It should be noted here that if the exhaust gas expansion chamber is integrally formed in the case member in the outboard engine system, the following problem is encountered: It is necessary to change the design of the entire case member or to replace the entire case member, which is a large-sized part, in order to regulate the silencing effect, resulting in a remarkable increase in cost.
The case member in the outboard engine system is generally comprised of a cylindrical extension case, a mount case coupled to an upper end of the extension case to support an engine block, and a gear case coupled to a lower end of the extension case. If the exhaust gas expansion chamber is integrally formed in the case member, it is necessary to disassemble the case member for the purpose of carrying out the maintenance of the exhaust gas expansion chamber. However, the following problem is encountered: The cases forming the case member are large-sized parts each having a large weight and moreover, are supported on a mounting bracket for supporting the outboard engine system on a hull through an elastic mount device. For this reason, to separate the cases, an extremely troublesome operation is required, resulting in a reduction in maintenance property.
Particularly, if the outboard engine system includes a 4-cycle engine, and an oil pan is provided within the case member, the following problem arises: The oil pan and the exhaust gas expansion chamber interfere with each other and thus, it is difficult to sufficiently ensure volumes of the oil pan and the exhaust gas expansion chamber.
An outboard engine system is known from Japanese Patent Application Laid-open No.8-312365, which includes a catalytic converter mounted in an exhaust passage provided in a case member for purifying an exhaust gas. In this outboard engine system, the catalytic converter includes an upstream introducing exhaust pipe and a downstream discharging exhaust pipe, and a mounting flange at an upper end of the introducing exhaust pipe is fixed by bolting within the case member.
It should be noted here that in the outboard engine system described in Japanese Patent Application Laid-open No.8-312365, the case member is comprised of a cylindrical extension case, a mount case coupled to an upper end of the extension case to support an engine block, and a gear case coupled to a lower end of the extension case. The catalytic converter is accommodated within the extension case and hence, to subject the catalytic converter to the maintenance, it is necessary to separate the mount case from the extension case. However, the following problem is encountered: The mount case and the extension case are large-sized parts each having a large weight and moreover, they are supported on the mounting bracket for supporting the outboard engine system on a hull through an elastic mount device. For this reason, an extremely troublesome operation is required to separate the mount case and the extension case from each other, resulting in a reduction in maintenance property.